D2.3 HL Water Relations in Plants
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What does water potential (Ψ) measure?
Tendency of water to move by osmosis.
What is the symbol for water potential?
Greek letter Ψ (psi).
What is the water potential of pure water?
Zero (highest possible).
What happens to Ψ when solute is added?
Becomes more negative.
In which direction does water always move?
From less negative Ψ to more negative Ψ.
What are the two components of Ψ?
Solute potential (Ψs) and pressure potential (Ψp).
What is solute potential (Ψs)?
Reduction in Ψ due to solutes (always negative).
What is turgor pressure?
Pressure from cell contents pushing on cell wall.
What is pressure potential (Ψp)?
Hydrostatic pressure from the cell wall; usually positive.
What happens when a cell is fully turgid?
Ψp balances Ψs, no net water movement.
What happens when a cell loses water?
Turgor decreases, cell becomes flaccid.
What is plasmolysis?
Plasma membrane pulls away from cell wall after extreme water loss.
What is the formula for Ψ?
Ψ = Ψs + Ψp.
Is full plasmolysis reversible?
No, it is irreversible.
What is the formula for Ψs?
Ψs = -iCRT.
Q18. What is i for NaCl?
2 (Na⁺ + Cl⁻).
What is i for sucrose?
1.
What does “i” mean in Ψs = -iCRT?
Ionization constant (particles formed when dissolved).
What does “C” mean in Ψs = -iCRT?
Molar concentration (mol/L).
What does “R” mean in Ψs = -iCRT?
Pressure constant (8.314 L·kPa·K⁻¹·mol⁻¹).
What does “T” mean in Ψs = -iCRT?
Temperature in Kelvin (273 + °C).